1. Introduction
Clean and renewable energy sources are in high demand due to current energy shortages and environmental concerns [
1,
2]. Supercapacitors (SCs) with a high power density and long cycle life with minimum pollution have attracted considerable interest [
3] because of their intriguing advantages, for example, lightweight and outstanding power, which are in high demand in manufacturing smartphones and handy gadgets [
4]. SCs usually store electrical charge on the electrode material’s surface by adsorption/desorption, known as electrochemical double-layer capacitors (EDLCs). In the second type, the near-surface area via reduction/oxidation states changes, and a reversible redox reaction occurs; this charge-storing mechanism is known as a pseudocapacitive mechanism [
5]. Pseudocapacitors (PCs) have a higher capacitance and energy density than EDLC-based SCs, but it is necessary to upgrade PCs before they can be used in real-world applications [
6]. The redox reactions governed by the electrode material’s multiple oxidation states significantly impact the performance of PCs [
7]. Designing nanostructured electrode materials for PC with high redox-active sites and improved performance is challenging. In this context, interchanging more than one electron for every redox reaction is considered to be advantageous. The performance of PCs can be improved by preparing the multiple valance state materials with abundant electroactive sites [
8]. Transition metal oxides have sparked interest in PCs over the last decade due to easy fabrication and cost-effectiveness [
4]. Binary metal oxides possess more valance states and higher electrical conductivity than single metal oxides, up to two times higher, and exhibit better performance [
9]. The poor performance of transition metal oxides may be due to their inability to conduct electricity. Following prior research, increasing the electrical conductivity of host oxides by do** them with metals or semi-conductor elements is essential for achieving high power density in SCs [
10]. Thus, Zn do** significantly increases the electrical conductivity of ZnCo
2O
4, especially in comparison with Co
3O
4, and redox-inert Zn
2+ offers good synergistic impacts and considerably improves the rate-performance of cobalt-oxide.
Cobalt-based binary metal-oxide electrode materials such as ZnCo
2O
4, FeCo
2O
4, CuCo
2O
4, NiCo
2O
4, and MnCo
2O
4 can significantly increase the performance of PCs [
11,
12,
13]. It is believed that ZnCo
2O
4 is the most promising candidate due to the low toxicity of zinc metal and reasonably significant capacitance of Co
3O
4 (3560 F·g
−1), and it also possesses good stability at low cost [
14]. However, the rate performance of cobalt-based binary metal-oxides is poor, and for SCs, the rate–performance parameter is critical. To enhance the ZnCo
2O
4 electrode’s overall performance, three general strategies are typically employed: (i) development of nanostructured materials with a visually appealing structure and a large surface area; (ii) research and development of new composites with high conductivity, which includes carbon nanotubes and graphene; and (iii) enhancement of the SC device’s potential window, as energy density is inversely proportional to the potential window’s square (E = 1/2CV
2) [
15]. Increasing the capacitance and voltage of SCs can improve their energy density. Capacitance can be enhanced by increasing the electrode material’s surface area and pore size [
16]. Binders prevent active materials from falling off during electrode operation by cohering the electrode material with substrates. When making an electrode, the binder should be able to provide the necessary strength and pore sizes. However, binding or cohesive agents inevitably cover some of the active material’s surface area or pores. As a result, the electrochemical performance of SCs is directly influenced by the binders and their content in the electrodes [
17]. Binder-free electrodes have recently been found to significantly increase electrical conductivity and ionic diffusion paths and thus improve overall performance. In addition, the “dead volume” in the electrode material can be avoided using a binder-free electrode with a highly porous and robust construction [
18].
Saravanakumar et al. fabricated a binder-free hierarchical electrode of ZnCo
2O
4, showing a suitable specific capacitance of 92 mF·cm
−2 at 0.5 mA·cm
−2 [
19]. Fu et al. made a binder-free flower-like ZnCo
2O
4 electrode with a good capacitance of 689.4 F·g
−1 at 1 A·g
−1 with excellent cyclic stability of 97.1% after 1500 cycles [
20]. Kamble et al. used ZnCo
2O
4 to make binder-free electrodes and obtained a good capacitance of 127.8 F·g
−1 at 1 mA·cm
−2 with excellent cycling stability of 3000 cycles [
21]. Although considerable efforts to increase the capabilities and cycling stability of ZnCo
2O
4 have been ongoing, the cycling stability and rate performance are still crucial and need to be significantly improved. The electrochemical performance of ZnCo
2O
4 electrodes can be further improved by exploring the various morphologies. We propose that electrodes of ZnCo
2O
4 with micro/nano morphology can increase surface area and pore size, resulting in increased energy density and rate performance.
Herein, we proposed binder-free ZnCo2O4 micro-flowers composed of nanosheets supported on (ZnCoO@NF), improving the rate capability up to 91.8% when current varied from 2 to 20 A·g−1. The ZnCoO@NF electrode exhibits a superior capacitance of 1132 F·g−1 at 2 A·g−1 with excellent cycling stability of ~99% after 7000 cycles at a high current density of 20 A·g−1. The improved performance of the ZnCoO@NF electrode is attributed to the highly stable structure of micro/nano-architecture, which provides both high electronic conduction and fast ionic transportation paths.
3. Results
Figure 1 gives a schematic route for the fabrication process of ZnCoO on nickel foam. In order to improve the mechanical and crystalline, the ZnCoO was post-annealed for the short term at 250 °C for 2 h in an air atmosphere with a ramp rate of 2 °C·min
−1.
Figure 2a–c shows the morphology of as-prepared ZnCoO@NF at different magnifications, reflecting the 2D micro-flowers made by nanosheets.
Figure 2a shows a low-resolution SEM image, demonstrating the rough surface with the growth of high-density ZnCoO nanosheet architecture on the entire surface of the NF and reflecting a compact design and interconnected structure. The high-magnification SEM images show that the various nanosheets make a micro-flower-like structure (
Figure 2b). An open channel structure formed by interconnected nanosheets and an average thickness of 38 nm is shown in
Figure 2c. Compared and a pure nanosheet, these micro-flower patterns increase the surface area of the electrode and enhance the electrode/electrolyte interface, making it better suited for use in SCs.
TEM analysis was further conducted to investigate the ZnCoO structure and surface morphologies in more depth. A characteristic TEM image with low resolution represents nanosheets of ZnCoO (
Figure 3a,b).
Figure 3a shows that a 2D structure can be derived from the interconnected nanosheets, increasing the intrinsic electronic conductivity.
Figure 3b shows that the lattice fringe spacing is 0.23 nm, corresponding to the 111 planes of ZnCo
2O
4 (PDF# 23-1390). These findings are also in line with the results of the SEM. It should be noted that in the SEM and TEM images, we can see some residual nanoparticles. It is confirmed that there is no effect from these residual nanoparticles on the electrochemical performance of the electrode.
Figure 4a shows the specific surface area and pore size distribution (PSD) of the ZnCoO@NF electrode recorded at 77.5 K. BET absorbed volume of ZnCoO@NF is about 50 cm
3/g at 1.0 P/P
0, which means that ZnCoO@NF has a highly porous structure (55 m
2·g
−1). Moreover, this study demonstrated an H1-type hysteresis loop, which suggested the presence of a mesoporous structure in the solution. The PSD was calculated from the desorption isotherm using the Barrett-Joyner-Halenda (BJH) method, as shown in
Figure 4b. The PSD of ZnCoO@NF is centred at about 9.6 and 61 nm, reinforcing the prepared sample’s mesoporous structure with pore sizes ranging from 9.6 to 61 nm. These findings show that the ZnCoO@NF sample has a greater surface area that leads to efficient ion and electron ship**, implying that high electrochemical activity in the ZnCoO@NF electrode is expected. For efficient PC electrodes, a high specific surface area can effectively enhance the electrochemical performance and provide a large number of electroactive sites for PCs.
The XRD pattern of ZnCoO@NF (
Figure 5a) is well-indexed according to the standard spinel phase ZnCo
2O
4 (JCPDF# 23-1390). The particular diffraction peaks of ZnC
2O
4 located at 21°, 32°, 37°, 39°, 44.7°, 56°, 59.5°, and 66° correspond, respectively, to 111, 220, 311, 222, 422, 511, and 440 lattice planes. Compared with the standard ZnCo
2O
4 spinel, these results are consistent with the previous findings [
20]. The lattice plane of 400 at 44.7° is overlapped with the XRD peak of NF. No other impurities are detected except two intense peaks from the NF at 45° and 52.5° of 2θ, demonstrating the high purity of the sample. The JCPDF# 23-1390 of pure Co
3O
4 was added as a comparison to confirm the difference between ZnCo
2O
4 and Co
3O
4. The diffraction peaks of as-prepared ZnCoO@NF clearly matched with JCPDF# 23-1390 and confirm the pure phase of ZnCo
2O
4. The crystal structure ZnCo
2O
4 nanostructures are shown in
Figure 5b. Binary spinel metal oxide ZnCo2O
4 with Co
2+ at the tetrahedral sites (8a) in the spinel Co
3O
4 replaced by Zn
2+ offers a better stability. In addition, the spinal structure can help increase the redox reaction sites, which boosts the specific capacitance of bimetallic oxide-based electrodes. Thus, choosing the bimetallic oxide and crystal structure is critical for overall performance and structural stability during the electrochemical discharge process [
22].
The Raman spectrum further characterized the as-synthesized ZnCoO@NF to confirm the Raman band characteristics. From
Figure 5c, the Raman spectrum of ZnCoO@NF demonstrates four active vibrational bands located at 183, 472, 513, and 682 cm
−1 assigned to the characteristic F
2g, E
g, F
2g, and F
2g phonon modes, respectively. The presence of Zn, Co, and O in these distinct peaks of the Raman spectrograms suggests the formation of ZnCo
2O
4. This indicates that the prepared sample is free of contaminants [
23].
The electrochemical performance results of ZnCoO@NF are shown in
Figure 6. The CV was performed at various scan rates in a potential window ranging from 0.0 to 0.5 V. The CV curves indicate two redox peaks at approximately 0.38 and 0.42 V (vs Ag/AgCl), which represent the typical pseudocapacitive behavior of the ZnCoO@NF associated with Co
3+/Co
2+ during the charge/discharge process [
19]. The reduction peak shift is related to the internal resistance of the electrode materials, which further confirms the pseudocapacitive features with enhanced charge storage [
24]. Moreover, the CV possesses a large area under the curve with high current response, indicating better charge storage behavior. The electrodes’ excellent redox reversibility can be seen in the redox peaks caused by reactions involving Co–O/Co–O–OH [
25].
The electrochemical property of the ZnCoO@NF electrode was tested based on charging and discharging at various current densities to accurately estimate the charge storage capability. The GCD measurements of the ZnCoO@NF electrode were carried out in a potential window ranging from 0.0 to 0.45 V.
Figure 6 depicts typical GCD curves of ZnCoO@NF at various current densities (2 to 20 A·g
−1) (b). The GCD curves are nearly symmetric (Coulombic efficiency 98%), have apparent redox behavior, and agree well with the CV results, demonstrating the pseudocapacitive charge storage features. The charge and discharge curves primarily have voltage terraces at about 0.3 V and 0.36 V, respectively. Equation (1) can be used to calculate the specific capacitance Cs based on the curves. Based on Equation (1), the obtained specific capacitances of the ZnCoO@NF electrode are about 1132, 1120, 1100, 1060, and 1040 F·g
−1 at current densities of 2, 3, 5, 10, and 20 A·g
−1, respectively, as shown in
Figure 6c. When the discharge current density changes from 2 to 20 A·g
−1, the capacitance remains at 91.8%. The capacitance of the ZnCoO@NF electrode is much better than other electrodes such as Gd-doped CeOx nanoflowers (280 F·g
−1 in 1 M NaOH at 1 V·s
−1) [
26] and mesoporous Co
3O
4 nanosheets on carbon foam (106 F·g
−1 in 1 M NaOH) at a scan rate of 0.1 V·s
−1 [
27].
Because of its multi-channel porous nature and enhanced ionic transportation, the ZnCoO@NF demonstrated exceptional specific capacitance, far exceeding that of previously reported ZnCo2O4-based electrode materials.
The total capacitance of ZnCoO@NF can be attributed to the following significant contributions: (1) the micro-flowers composed of nanosheets with highly open structures supply abundant active sites for the Faraday reaction and offers easy diffusion paths for fast ionic transportation; (2) the electrochemical performance can be promoted by improving the specific area, pore size, and multi-channel nano/micro-architecture.
Furthermore, electrochemical impedance spectroscopy (EIS) was conducted to study the kinetics of charge transfer in the ZnCoO@NF electrode to confirm the charge transport properties.
Figure 6d displays the Nyquist plots of the EIS spectra for the ZnCoO@NF electrode. The diameter of the semi-circle is used to estimate the charge transfer resistance (R
ct = 1.52 Ω), which means that ion diffusion and electrolyte penetration in the active materials are reduced. The solution resistance (R
s = 0.13 Ω) is the combination of the electrolyte’s and electrode’s internal resistance. The values of R
ct and R
s are smaller than in pristine Co
3O
4 (R
s = 1.32 Ω) [
28] and ZnO (R
s = 2.82 Ω and (R
ct = 2.25 Ω) [
29]. The high-sloped curve at the high-frequency region indicates the fast ionic diffusion with enhanced electroactive sites in the ZnCoO@NF electrode material. The Nyquist plot was fitted with an equivalent circuit using ZSim view software, as presented in the inset of
Figure 6d. The equivalent circuit shows R
s, R
ct, C (the double layer capacitance), and W (the Warburg diffusion element). Hence, the ZnCoO@NF electrode possesses more capacitive-type charge-storage features [
30].
Another vital parameter to consider when evaluating the practical applications of SCs is their cycling stability. Thus, the cycling stability test of the ZnCoO@NF electrode was performed by deliberately running it for 7000 charge/discharge cycles at a high current density of 20 A·g
−1, and the results are shown in
Figure 7a. After 7000 cycles, the specific capacitance remains at 99%. In the long-term cycling test, the ZnCoO@NF exhibited excellent charge stability and high performance, as shown by the obtained results. Moreover, at 20 A·g
−1, the first and last five GCD cycles are shown in
Figure 7b, indicating that the ZnCoO@NF electrode exhibits excellent stability before and after testing. Due to the large exposed surface area of its unique micro-flower composed of nanosheet architectures, the ZnCoO@NF electrode has impressive electrochemical performance. Micro-flowers can be grown directly onto the substrate without a binder, enabling direct electron intercalation between the substrate and the active material. It is easier to conduct a fast ion-diffusion reaction with the active material due to the micropores in the NF substrate. Because of their high rate capability, excellent capacitance, and admirable cycling stability, ZnCoO@NF electrodes are potential candidates for pseudocapacitive SC applications. The detailed comparisons of the current work with the previous literature in terms of electrode material, capacitance, current density, number of cycles, and retention are presented in
Table 1.